Field of the Invention
The present invention relates to methods for improving registration between images, for example images generated by different imaging modalities. It particularly relates to the registration of medical images, typically the registration of an anatomical image with a functional image.
Description of the Prior Art
The present invention particularly relates to the registration of such images in respect of essentially elongate features, and more especially to such images of ribs of a patient.
As used within the present document, the following abbreviations may carry the following meanings:                CT Computed tomography        PET Positron emission tomography        SPECT Single-photon emission tomography        MIP Maximum intensity projection        SUV Standardized Uptake Value        18F-NaF 18F-Sodium fluoride        99mTc Technetium-99m        
Typically, CT and PET imaging data are acquired together, in a single dual-mode scanner. Anatomical information gained from the CT data can be supplemented by functional information provided by the PET image data. Therefore, combining the data from both sources may increase the diagnostic conclusiveness of the images. The CT image may be used to estimate the attenuation effects in the PET image, and to guide some compensation for these attenuation effects in later image data analysis.
The CT data acquisition may be performed before the PET data acquisition begins. Typically, a CT image is generated by data acquired over a few seconds, while a PET image represents data acquired over several minutes. It is not possible for the patient to remain immobile for the length of time taken to assemble the PET data, at least due to the motion involved in breathing. The misalignment will be of the order of a few millimeters. This will typically be an insignificant misalignment when looking at relatively large features of a patient, such as the heart or lungs. However, when imaging finer features, such as ribs or vessels for example, such misalignments may become significant. Hence, this inherent “scanner alignment” provided by the hardware arrangement has known limitations.
It is possible to attempt to realign the images with respect to one another to reduce the misalignment of features, by relatively moving one image by way of translation and/or rotation, for example. Such “global alignment” may provide a useful reduction in misalignment for structures whose internal configuration remains identical in both images.
However, for fine features such as ribs or vessels, the features are to some extent moveable with respect to one another, so the relative positions of the features in one of the images may be different from the relative positions of those features in the other image. It is then difficult to achieve acceptable alignment in the inherent scanner alignment. It is also difficult to improve the alignment between the two images by global alignment, as a translation and/or rotation which provides an improvement in the alignment of one rib, for example, may in fact degrade alignment of another rib.
Although one image may be scaled and distorted to provide a better fit to the other, it is preferred not to distort medical imaging data, as the required information may be distorted or lost.
While the present invention may be applied to elongate anatomical features other than ribs, for example the spine, or vessels, the following description will make especial reference to ribs, in the interests of brevity and clarity.
It is known that images of ribcages may be “unfolded” to generate a more-easily understood 2-dimensional image from 3-dimensional anatomical data. WO2006/050102 describes the “unfolding” of CT imaging data relating to the ribcage. The CT imaging data provides anatomical information: information relating to the bone structure of the ribcage. The centerline of each rib is extracted. Then, for each rib a 2D projection of its centerline is visualized. Lesions and corresponding rib information are easier to determine in such an “unfolded” image than in the original 3D image data.
While CT and similar “anatomical” image data usefully indicate the bone structure of a patient, other “functional” imaging modalities such as PET or SPECT are more effective at detecting lesions within the bones. On the other hand, such functional imaging modalities are less effective at detecting the bone structure. Bone metastases are common with certain cancer types, and the present invention relates to improved methods for identifying and locating such features. Throughout the present description, the term “PET” will be used to refer to PET or SPECT, in the interests of brevity.
US2013/0101197 defines an improved method whereby “unfolded” images may be generated, which combine the anatomical information with the functional information. By segmenting the functional image data according to the rib locations identified by the anatomical image data, separate images of each rib may be generated. These may in turn be projected along the calculated centerline of each rib to provide a single 2D image representing a MIP (maximum intensity projection) of the functional data overlaid on an average value of the anatomic data. Such combined images make the identification of affected ribs relatively easy.
However, this method has been found to suffer from certain drawbacks.
The alignment between the representations of the ribs in the functional image may not sufficiently correspond to the alignment of the ribs in the anatomical image. Although the ribs themselves may be considered solid, and may be assumed to have retained a same shape between the two images, their relative locations may be different between one image and the other. The anatomical image and the functional image will typically have been captured at different times, even if those times are typically quite close together. The patient's breathing, or other movement, may cause relative movement between the ribs, and may cause the ribs to be in different relative positions in the functional image than they were in the anatomical image.
When one uses the method of US2013/0101197, the shapes of the individual ribs identified in the anatomical image are used to identify parts of the functional image which represent corresponding ribs. Where the ribs are in different relative positions in the two sets of image data, the parts identified in the anatomical data will not correctly align with the representation of the ribs in the functional data.
The described rib centerline detection is usually applied only to the CT data. Combining CT with corresponding PET data would allow better identification of potentially cancerous areas of bones than is possible using only CT data. The time-averaging effect of the long PET exposure time may result in significant differences between the position of each rib in the PET and CT images. This is illustrated in FIGS. 1A and 1B. FIG. 1A shows detected centerlines 10 on the original CT image 15, while FIG. 1B illustrates the same centerlines 10 mapped onto the corresponding scanner-aligned PET image data 20. Using the CT centerlines 10 to create an unfolded view of the PET image 20 may accordingly result in a severely misaligned PET projection.
Inaccuracies in image alignment may complicate the visual correlation of functional (e.g. PET) and anatomical (e.g. CT) information in regions of interest or impede an unambiguous anatomical localization of regions with increased PET tracer uptake. This may lead to the resultant combined image indicating a lesion in an incorrect position, on the wrong rib, or not identifying it at all.
The present invention allows improved alignment between data representing the same rib in both the anatomical image and the functional image.
The present invention provides an alignment method which is robust to variations in PET scanning protocol, such as the use of a reconstruction method, or a post-filter. To increase its reliability, PET data can be pre-processed using an image filter to compensate for different effects caused by the scanning procedure used. This may provide a more consistent appearance in data from different scanners or reconstruction protocols. Such pre-processing steps are known in themselves, and known methods may be used, such as those known as an algorithm named SUVref, described in US2010/0290680. SUVref allows lowering of the effect of a reconstruction protocol and an increase in comparability of PET images that have been reconstructed with different parameters. It processes the images by applying a reconstruction-dependent Gaussian smoothing.
A technique for the automatic extraction of rib centerlines has also been proposed in US2013/0077841 or Wu, D., et al., “A learning based deformable template matching method for automatic rib centerline extraction and labeling in CT images” IEEE Conference on Computer Vision and Pattern Recognition (CVPR), 980-987 (2012).
Known techniques for functional imaging of bone lesions uses 18F-NaF PET/CT or 99mTc SPECT/CT imaging (Even-Sapir, E., Metser, U., et al., Journal of Nuclear Medicine 47(2), 287-297 (2006)).